Aspects of construction of safety mats

ABSTRACT

An array of resilient floor tiles is assembled into a continuous sheet after being laid down. An array of included, sacrificial resistive wires is buried along the edges of the tiles and is controllably heated in order to cause welding of the edges of tiles across the paths of the wires to neighbouring tiles. Subsequently the wires may be used to give the array integral tensile strength. The welded array is provided with greater strength for resisting use, expansive and contractile forces caused by environmental heat and cold and also long-term tile contraction owing to loss of plasticiser as may be seen with PVC-based tiles.

FIELD

This invention relates generally to apparatus and methods for assembling thermoplastic or elastomer materials, and to construction of resilient playgrounds or safety mats made of tiles. In particular the invention refers to means for assembling and co-adhering an array of laid-down tiles made of a poly vinyl chloride plastics composition.

BACKGROUND

This invention relates to resilient playground or floor (safety) mats, as embodied in the applicants' products most of which are made up of arrays of square tiles, 0.5 metre along an edge, made of a resilient material such as polyvinyl chloride (PVC) cast in moulds including perforations and surface detailing, so that the tiles have improved grip and resilience properties. Such PVC mats may include plasticisers, fillers, and colouring material and may include conductive material. Mats are intended to lie flat on a substrate and be attached by their edges to neighbouring tiles. Interlocking complementary lugs and pockets are of assistance. It has been found that more extreme environments such as outdoors in North America have been causing unexpected expansion/contraction of playground mats as a result of temperature changes. In addition a loss of dimension over a period of years is known to occur because of the gradual loss of the significant amount of plasticiser incorporated with the plasticiser. (Typically, about 20% by weight of flexible PVC is a plasticiser such as dioctyl phthalate. This substance can diffuse through the mass of the PVC). As a result, the glued joints become infiltrated with plasticiser. The inventors have realised that glued joints can be expected to fail when the plasticiser migrates into the glue itself, over time, and causes weakening of the joint. As a result, tiles lift off the substrate and/or separate from each other. Of course, the invention is in no way limited to use with the applicant's tiles; the above problem has merely stimulated the research that resulted in this invention. Many other articles constructed from plastics are amenable to a welding treatment according to the invention, as will shortly be described.

PROBLEM TO BE SOLVED

The problem to be solved could be stated as providing a permanent join between parts made of a plastics material where glued joins have been found to be weak and of short life. The inventor's particular desire is to halt any tendency of tiles to lift off the substrate and/or to separate from each other. One response would be to render the body of the tiles less prone to expansion or contraction, but as yet no economical solution having this effect has been found. Another solution is to provide better adhesive procedures including use of melted and then flowed-together plastics material; that is, welds. A further solution is to include tough fibrous material that traverses parts such as an array of tiles, having sufficient tensile strength to hold the tiles in place despite the forces of contraction or expansion.

PRIOR ART

U.S. Pat. No. 6,520,790 (Sumitomo Wiring Systems) teaches a method for waterproof lighting fixture assembly in which wiring is at first continuously routed through one or more sockets and then after a heat-welding process using resistive heating of the wire that melts a sealing material on to the wiring, the wires are cut at the sockets so that the same wire can supply power to lamps inserted in the sockets. This is a dual use of the same wire; first in an assembly phase then in a normal-use phase.

U.S. 6,676,861 Van Der Werf et al suggests that “the wire” as we later define it could be coated with various granules including coloured granules, for joining linoleum. The coated non-metallic cord of this invention does not include means for internal (resistive) heating.

There is a known technique in which a wire embedded in rubber windscreen mouldings of automobiles is heated in order to vulcanise the rubber. The wire is left in place.

OBJECT

It is an object of this invention to provide arrays of safety floor tiles or particularly play mats having a better capacity to stay united, or at least to provide the public with a useful choice.

STATEMENT OF INVENTION

In a first broad aspect this invention provides means for construction of an array of thermoplastic safety floor mats or play mats upon a substrate or surface; wherein heating means capable of causing the thermoplastic material to soften at least in a localised area is employed to cause the mats of the array to become linked or welded together along their edges, so that use of a glue is avoided.

In a first related aspect this invention provides means for the joining together by welding of an array of safety floor mats or play mats by their edges upon a substrate or surface; wherein heating means consisting of an electrically conductive elongated member having tensile strength is placed along at least one edge of a first tile between an upper surface and a lower surface of the tile and adjacent a second tile to which the first tile is to be joined, the adjacent edges comprising what is or will become a seam; the elongate member being capable of being heated by the passage of electric current in order to cause welding together of the first tile and the second tile along the seam.

In a second related aspect this invention provides an electrically conductive elongated member comprising a wire comprised of a metal or a metal alloy, having tensile strength and electrical conductivity.

Preferably the elongated member is composed of a stainless steel wire.

Alternatively the wire can be comprised of other conductors having tensile strength, including “Nichrome” alloys.

More preferably the wire includes at least two stainless steel wire strands twisted together, so that the combination produces a given amount of heat at a higher current but lower voltage yet retains more flexibility than a single thicker wire.

Optionally the wire is coated with compounds capable of enhancing the welding process.

Optionally the wire is coated with materials capable of increasing the effective diameter of the wire so that there is less tendency for the wire to pull or be pulled from its location.

Alternatively the wire is configured so that its surface includes projections.

Optionally the wire may be laid in a pattern which traverses more than one length of tile-tile weld, or intended weld, by returning to the source along the same or a different folds back onto itself.

Optionally, the wire is provided with a number of short sleeves of an insulating material so that perpendicular cross-over of wires does not carry current. One material suitable for a sleeve is shrink-wrap insulation.

In an alternative aspect the initial or a remedial welding process is provided by an alternative heat source: the local application of heat along the seam from a source of energy in contact with a surface overlying the seam.

Preferably the source of energy comprises a source of heat within a metal block.

In a second broad aspect the wire, which remains in place, is used also as a tensile reinforcing member within the array of safety floor mats or play mats by connecting each end of a length of wire to an anchoring means attached to the ground, each anchoring means being capable of holding the wire under a controlled amount of tension.

Optionally at least some of the tiles are provided with longitudinal edge grooves for holding the wire along their edges, although ungrooved tiles are satisfactory.

Preferably enough wires are included in an installed array of tiles to surround all edges of each tile of the array with a weld-capable retaining wire.

In a third broad aspect the invention provides apparatus for supplying the elongated conductive members with a welding current; the apparatus comprising means for generating the delivered current and controlling the delivered current.

Preferably the apparatus includes means for indicating the delivered current and for controlling the time of delivery and displaying the subsequent recommended cooling time, and connecting means for joining more than one wire to the current supply apparatus.

In a fourth broad aspect the invention provides a method for providing a more secure array of resilient tiles for play and/or safety purposes, the method including the steps of:

-   -   1. laying the array of tiles upon a prepared surface, optionally         but not often including an underlying adhesive;     -   2. laying lengths of the wire across the array, within every         inter-tile groove;     -   3. laying lengths of the wire along the array, within every         inter-tile groove;     -   4. in both cases leaving sufficient protruding wire at both ends         for electrical connection to be made and for subsequent ties to         be made;     -   5. optionally adding weld-enhancing materials,     -   6. applying an electric current from one end of each wire to the         other in order to cause resistive heating for the wire         sufficient to reach a welding temperature, and holding the         temperature for an effective duration, so that adjacent tiles         become welded by their edges to adjacent tiles,     -   7. allowing the weld to cool, and then optionally . . .     -   8. exteriorising the ends of each wire     -   9. coupling each free end of the wire to a retainer attached to         the prepared surface or the adjacent ground and applying         controlled tension to the coupling.

In a fourth broad aspect the invention provides a group of means for the local application of heat along two adjoining edges, so that the edges are raised to a welding temperature of between 130 and 190 degrees Celsius, and then brought firmly together so that the edges fuse and remain fused after cooling down.

Preferably the group of means for local application of heat includes hot air blowers, heated contact elements, ultrasonic energy, radio-frequency heating (including microwave heating), and infra-red heating; all adapted for the application of heat to a localised strip along the edges of adjoining items made of a poly vinyl chloride plastics material; and used either alone or in combination with other means.

Preferably the group of means for local application of heat are used in conjunction with head localising devices including air controlling barriers, ultrasonic transducers of controlled shapes, radio frequency electrodes, microwave applicators, and infra-red reflectors and directors, so that the heat is restricted to a localised area to be welded.

A preferred microwave frequency is in the range of either about 915 MHz or about 2.4 Ghz).

In a fifth broad aspect the invention provides for the use of a plastisol glue as defined elsewherein this specification, either separately or together with methods previously described in this section; the plastisol glue being first placed in the area to become welded, and then heated to a temperature sufficient to cause the sol to melt and become converted into a mass of poly vinyl chloride material.

Preferred heating means include the heating of embedded wires, hot air blowers, heated contact elements, ultrasonic energy, radio-frequency heating (including microwave heating), and infra-red heating;

PREFERRED EMBODIMENT

The description of the invention to be provided herein is given purely by way of example and is not to be taken in any way as limiting the scope or extent of the invention.

DRAWINGS

FIG. 1: is a diagram showing a section of a tile near one edge, including the wire ready for use in its first function of welding.

FIG. 2: is an illustration of a wire as used.

FIG. 3: is a plan view of one clamping device to force tile edges together while applying heat to the seam to be welded.

FIG. 4: is a diagram showing the layout of wire welding wires within a matrix of tiles.

FIG. 5: shows tying down of the wire after completion of welding, for the second function of holding tiles against the ground under some tension.

FIG. 6: a perspective drawing showing an over-centre action type of clip; a second device for forcing tiles together during welding by pegs that enter holes.

FIG. 7: is a circuit diagram for a power supply suitable for providing power to welding wires.

EXAMPLE 1A

The welding and holding wire central to this Example is laid along the seams between an array of flexible PVC-based tiles as sold by the inventors under the trade mark “PLAY MATTA” when laid down for use upon a substrate. The wire has two main purposes: (1) for welding use during installation, when it provides the localised intense heat for a mat welding procedure, and (2) for use after installation when the tensile properties of the wire are used to hold the array of tiles together, and also down on to the underlying substrate despite forces tending to pull the tiles away; including thermal expansion and contraction, contraction on volatilisation of plasticiser, and applied forces.

The applicant usually provides tiles either in single units (0.5 metres square), or as one-metre pre-welded squares each holding four tiles (size information by way of example only).

FIG. 1 shows in section the abutting edges of two safety tiles 101, 102 each of which are made of a thermoplastics material such as PVC (polyvinyl chloride) including about 20% dioctyl phthalate or equivalent as a plasticiser, with colorants, fillers, blends and other additives. (That is the plastics material assumed in this illustrative and non-limiting example of the patent). Some kinds of tiles may also include complementary lugs and pockets along their apposed edges The two tiles of FIG. 1 have been welded together by heat generated by resistance to a flow of electricity passed through the wires 103, 104 (preferably a twisted pair) causing melting and later resolidifying of the surrounding area 105. An option of a groove between the tiles, possibly useful for pre-locating the wires, can be dispensed with, especially if compression is applied across the joint during the welding process so that the space occupied by the wire before heating is taken up by their melting into the bodies of the adjacent tiles. Usually, an installer will insert the wire into the space after the tiles have been first placed on to the substrate, probably inserting it with the help of a depth-determining tool like a miniaturised mole plough. It is useful for the wire to be laid straight, without kinks and at a reasonably constant height so that tension can subsequently be applied.

A desired welding process has a cycle length of 5 to 8 minutes at a power input in the range of about 100-150 watts per metre. The actual temperature reached has not been measured, but is less than the decomposition temperature for PVC and is probably 140-180 deg C. The power supply of Example 1E may be adjusted in order to provide this amount and duration of heating power.

EXAMPLE 1B The Wire

FIG. 2 a shows detail of the wire (200) used. The presently preferred wire is a stainless steel wire, preferably a soft-temper 304 alloy stainless steel, supplied as a twisted pair (strands 201 and 202) of wires each having a preferred thickness of about 18, 20 (about 0.88 mm diameter), or 22 gauge is preferred. (Other alloys such as 316 may be used as substitutes, and less or more than two strands could be used.) A greater number of strands of wire is more flexible than a lesser number for a given total cross-section. Single strand wire is an acceptable equivalent. Stainless steel (one of several alloys of mainly iron with chromium) has a relatively high electrical resistance compared to copper. Resistance wire or “Nichrome” (a trade mark of Driver-Harris Company, Chicago) is another option (one of several alloys of nickel with chromium) having about twice the specific resistance of stainless steel. Iron wire, or high-tensile galvanised (or otherwise zinc-coated) steel wire is not advised for use with PVC tiles because of the usual release of free hydrochloric acid from the plastics mass during heating of PVC to the working temperatures expected. The acid then reacts with the zinc coating and the underlying iron, which results in rust stains upon the tiles and destroys the wire's mechanical properties. Some alloys of stainless steel may also be relatively vulnerable to hydrochloric acid. Steel may be suitable for use with other weldable resilient plastics and elastomers.

An option of providing a thick coating of relatively meltable granules applied to the wire or to the tile edge before the welding process has been considered, but subsequent migration of plasticiser may weaken this material a while after it has formed an adhesive, and it comprises an extra step in manufacture over simply welding with the bulk material of the tiles. Perhaps tile pressure could be realised within a confining outer frame by weighting the slightly oversize tiles down. Clips are not essential.

EXAMPLE 1C Clip Apparatus for Closing the Seam

Temporary clips or other devices to pull the seam shut during the welding process and hold it shut in compression until the weld has cooled are desirable. One device is a pair of rows of lever-actuated pushing rods that engage with existing holes through the tiles, as shown in FIG. 3. A frame having a central clear aperture about 600 mm (25 inches) long is provided so that an iron (to be described later) can freely access all of the seam between two tiles (101, 102) to be welded without obstruction of the seam by clips.

The seam compressor shown in plan view (top view) in FIG. 3 is mounted on a rectangular steel base plate 300 having a rectangular aperture 300A that is about 600 mm long. This example device makes use of the holes 301 that are already included as an array over the surfaces of the tiles 101, 102. The device includes a peg 302 at the end of each member 303 which are first engaged in the holes in the tiles and then made to traverse towards each other across the seam 103 by forcing the pushing frame 304 to move across the surface of the plate 300 from right to left, as shown in FIG. 3. The pushing frame includes a plurality of inwardly directed pegs 305 (slope exaggerated) each of which pushes against a short vertical shaft 306 that is constrained to move only longitudinally by virtue of being engaged in slot 307. The pegs 302 are forced to move in an arc around pivot point 309 by being coupled to that pivot point by rigid beam 308. 303 and 308 may be regarded as a leaf hinge as seen from one end. All the pegs move in synchrony so that moving frame 304 to the left causes the seam 103 to be pushed shut. When shut it may be manipulated so that both sides lie at the same height, so that a 0.5 metre long heated iron as described later in this section can make contact with the entire length of the seam 103 between two tiles and compress and weld it. A series of seam compressors are placed along the full length of a seam to be welded—perhaps using 25 individual seam compressors. Means to cause frame 304 to move, forcefully, to the left my be as shown at 310 (the eccentric cam), 312 (a short vertical shaft which turns within a hole directly beneath, in the plate 300, and 312, a handle for turning the eccentric cam device; all shown from straight above. Such means could comprise (a) an eccentric cam mechanism fixed to the base place 300 (see later), (b) a “knee” type lever system, (c) a rotating advance screw. In practice (d) a hydraulic or air-driven ram might provide the movement in a convenient and economic manner. This device assumes that the resilient tiles themselves provide sufficient give (resilience) to take up minor differences in position.

If that is insufficient, beam 303 may be made of a springy steel that has a suitable amount of give for the expected force. This drawing illustrates the principles of clamping devices but has not resolved the matter of providing a single long clear aperture along perhaps up to 30 metres of seam. The device shown is more particularly suited to use of a hot “iron” rather than an internal heated wire. If an iron is not likely to be used, a simpler version of clamps for holding the tiles close together along the seam may be made. See the perspective drawing of FIG. 6. These example clamps (602, shown loose on a surface, and 601, shown tightened in place) use an over-centre action lever to hold the seam closed during welding between two steel pegs 606 and 607 which are inserted into holes 301A and 301 in adjacent tiles 101 and 102. (“Over-centre” as used herein means that the clip is held in place by being tightened beyond a peak value of tightness and is then held at a mechanical stop; the resilience within the tiles maintaining the clip in position). When the handle 605 is turned anticlockwise around the peg 606 that is welded to the handle, the far end of the curved steel rod 603 that comprises peg 607 is moved further from peg 606 because the near end is pivotally mounted through the handle 606 (emerging at 605 in the drawing). The rod 603 has been specifically dimensioned for use with the pattern of tiles used in 101 and 102. On tightening, the horizontal curve of rod 603 at 604 reaches around the axis of peg 606 and comes to a halt when the rod 603 contacts peg 606, just after having maximally compressed the tiles. The tile edges are compressed together around wire 201 that has been placed within the seam 103.

EXAMPLE 1D Apparatus for Heating the Wire: General

Apparatus for supplying the welding current is described here. Environmental conditions and working voltages and currents must be considered in relation to operator safety since potentially lethal amounts of current are used. A motor-driven generator may be preferred, being portable, inherently isolated from ground, and being a variable power supply. Isolating transformers and return-current detection circuit breakers are of assistance if the utility power is used. About 12 volts RMS per metre of seam, at a current within a range of about 5 to 20 amperes (depending on the resistance per unit length of the wire used) is required to be supplied for a duration of 5 to 8 minutes. A constant-voltage power supply is preferred over a constant-current type, using the negative temperature dependence of resistance of a metal for some auto-regulation. A mains-driven arc welder can be used if suitably calibrated. DC power is acceptable except that it is more difficult to control cheaply: because of factors such as switch derating and non-availability of transformer or phase-controlled rectifier (Triac) controls for instance. The power supply may be required to produce from 6 volts to, for a 20-metre span, over 240 volts (measured as RMS voltage across the length buried between tiles) depending on the length of seam between tiles to be welded at any one time. A selectable transformer tapping may be the most reliable way to achieve the variable voltage although solid-state devices are preferred options and are amenable to automatic control. The operator would be required to select the closest tapping or power setting for the wire length and type in use. The current should be delivered in a controlled manner over a period so that the result is melting and welding, rather than overheating and charring of the materials surrounding the wire (the welding wire) if too much current is used. The operator would be instructed never to overheat a seam—which would char it and require replacement of adjoining tiles. Underheating would fail to weld the tile material.

Ideally there would be a display device to show (a) the progress (time elapsed) of the welding cycle, (b) that current is flowing properly, and (c) the status of delivered power. A temperature probe may be used. Signalling the end of a suitable cool-down period at the end of a weld is also important, in case under-skilled workers try to rush the process and pull the retaining clips out too soon.

EXAMPLE 1E Circuit of Apparatus for Heating the Wire

This presently preferred circuit relies on an alternating-current source, such as a motor generator as would be used in the field (or alternatively use of a large isolating transformer) to ensure isolation of the power from the general ground, and hence safety. 230-250 volts is generally required to match power transfer to a desirable welding wire thickness, and length, rather than the perhaps more obvious 110 volts, for which wire thickness has to be increased significantly (hence raising cost and using a stiffer wire which is hard to handle). Continuously variable heating control is provided with little heat dissipation by phase control (using the circuit board (CB) and the phase shifting components mounted on it) of a “Triac” solid-state device. A snubberless type is preferred since it commutates better when driving an inductive load. A purely resistive load (R6) is also provided since this power supply includes a step-own transformer for better control over short lengths of wire. That second range of heating currents is made using the 6.6:1 stepdown toroidal power transformer (TT), for use on short lengths of welding wire. The controller is provided in a box having a lid. Some parts are mounted under the lid; others on the base. Meter measurements of delivered voltage and current are provided, and timer action is indicated by the two lamps. The circuit of FIG. 7, together with the accompanying list of components and this introduction shall be understood as a group by one of skill in power electronics.

The “Omron HC3A” timer ensures accurately repeatable yet variable-length applications of heating current by time, and the control R1 is used to apply an amount of heat according to the amount of resistive wire in use at any time, so that the workman's job is simplified. In effect, the circuit on the left represents means to energise the selected portions of the circuit on the right for a controlled period by the mechanism of relay coil power (coils SRC, and either LRC or HRC are energised). Conventional “TRIAC” triggering circuitry is provided along with a 50 or 60 Hz frequency selection switch.

Parts List A: Components Mounted on the Lid

-   -   P Input 230 Volts single phase AC; 50-60 Hz (such as from a         motor generator, effectively isolated from ground), or (with         proper safety considerations) across two phases of a 3-phase 117         volt supply).     -   MS Main switch     -   CB Circuit breaker (16 AmpC curve 10 kA)     -   ES Emergency switch—dual contacts     -   N Connection to a neutral bus (wiring between (N) is omitted for         clarity)     -   L1 Green neon light, indicating “power on”     -   L2 Amber flashing light indicating “heat is being applied”     -   SS Start switch—normally open     -   TS Test switch—normally closed     -   HLS High/Low power switch     -   SRC Signal relay coil     -   LRC Low power relay coil     -   HRC High power relay coil     -   Timer (Omron, Japan #HC3A, or equivalent)

B: Components Mounted on Base

-   -   OS Output socket (x 2); the welding wire is connected across         these.     -   R Resistance test point (x2); the welding wire resistance may be         tested across these.     -   SR Signal relay contact     -   A Ammeter (20 Amp O/S 40 moving iron AC)     -   V Voltmeter (250 Volt AC moving coil)     -   HR High power relay contacts (double pole)     -   LR Low power relay contacts (double pole)     -   FC Ferrite choke     -   TT Toroidal transformer (6.6:1 step-own/500 VA) for controlled         low voltage outputs.     -   T Triac (25 Amp—snubberless. Example: BTA06-600BW         (SGS-Thompson))     -   R1 Control potentiometer (500K)     -   R2 3K3     -   R3 1M(50 Hz setting)     -   R4 1M(60 Hz setting)     -   R5 15K     -   R6 1K5 (50 watt) This assists when driving inductive loads.     -   FS Frequency changeover switch     -   C 100 pFMylar(2 off)     -   D Diac     -   N One of a number of connections to a neutral (or one phase)         bus.     -   Notes: Two SR contacts for resistance testing are normally         closed. One SR contact feeding the circuit board is normally         open. Black dots represent stud terminals. “N” in a circle         represents each of a number of connections to a neutral bus.

EXAMPLE 1F Other Heating Methods

A: A hand-held hot-air blower might be used for local additional heating, such as for patching use after the main process has been completed. Since the air heats only the upper surface it would be usefully complemented by sub-surface wire heating.

B: The most straightforward of many alternative processes is thermal conduction from an internally heated metal block device (herein called “an iron”) which is a simple and effective method such as for patching a playground surface by welding if a tile needs to be replaced. It also heats only the upper surface of adjoining tiles. The compressing clip system would be used at the same time. In some cases, access to the welding wire may be inaccessible, or may not have been used. One must remember that the workmen need simple, reliable and easy-to-understand equipment. This description of a prototype iron is given by way of example. A rectangular bar of copper or aluminum, 25 mm×25 mm×0.5 metres (1 inch×1 inch×20 inches) is used. Copper is heavier and a better thermal conductor than aluminum. The bar is drilled out axially lengthwise in order to accommodate two resistive heater units each rated at about 500 watts at the local mains voltage (117 or 240 volts), or as generated by a motor generator. In this case electrical isolation between the heating element and the metal block is reasonably likely and can be checked by means of a residual current sensing device. Example heaters are 5 to 6 mm diameter. The bar is also milled lengthwise along what will become the top surface in order to accommodate the metal bulb of a thermostat, placed so that it samples an average of the temperature of the bar. (A thermocouple, read by a circuit within the power supply unit, is preferred for production units. Fail-safe means include testing for open circuited thermocouple wires and use of a backup thermocouple to sense over-temperature conditions. The bar is covered with insulating material on all but the base. A sheet mica material (Hislop & Barley, Onehunga, New Zealand) is used and a glass-reinforced “Teflon”® sheet covers the base through which sheet heat will be transferred to the underlying plastic in order to cause a weld. The Teflon allows release afterwards. An electrical circuit for use with an “iron” includes these features:

-   -   1. switch means to change between series and parallel connection         of the two heaters for faster initial heating then holding a         temperature;     -   2. thermostatic control of the maximum temperature (placed in         series between heaters and supply);     -   3. Optional timer to set the duration of the heating phase of         the cycle (placed in series between heaters and supply);     -   4. Fuse (placed in series between heaters and supply) and         indicator lights to show the current status of the iron.

EXAMPLE 1G Wire Laying Patterns

FIG. 4 (with tiles shown shrunken apart for clarity) shows in plan view an area of tiles in a playground having suitable layouts for laying the stainless steel wires in loops, in a first direction (201Y1, and (optionally) returned as 201Y2, or again in a second direction 201X1, (optionally) bent back at at 201XU, and returned at 201X2 in order to weld an array of tiles together with welded seams along all sides of the tiles. It will be noted that a layout should provides wire along all perimeters of all tiles.

Total wire length will be limited by the amount of power that can be inserted. Relatively long straight lines may be welded at one time, using one or more low-resistance insulated connecting leads to close the current path. Insulating sleeves such as heat-shrink sleeves may be slipped over the welding wires where they cross over other wires between the tiles in order to avoid bypassed current effects and to avoid local heating arising from poor contacts.

EXAMPLE 1H Tensioning

FIG. 5 shows how, once welding is completed, and the wires of FIG. 4 have been cut apart and exteriorised the freed ends can be fastened to retainers which are secured to the substrate or to adjacent fixed points. Alternatively a wire stretcher could be used during welding as well. Most playground installations place tiles 101, 102 etc on a substrate that has a timber, concerete or similar surround 503. This Example shows small turnbuckles 501 that allow the tension to be precisely set on individual strands, not so high that the material of the wire becomes stretched, and distorts the tiles, but high enough to prevent the tiles from creeping, rising above the contact with the substrate, or moving laterally. The turnbuckles are anchored into the timber or other surround by a peg, screw, or the like 502 and can be covered for safety.

EXAMPLE 2 Method

The preferred method for providing a more secure array of resilient tiles for play and/or safety purposes includes the steps of:

-   -   A. laying the array of tiles upon a prepared surface within a         timber or concrete frame so that the surface is covered, as         singles or as pre-welded groups (such as in squares of 2×2         tiles);     -   B. laying lengths of the wire across the array, within every         inter-tile groove, optionally using a depth setter/positioner;         (a total length to be welded at one time may be 25-30 metres         long);     -   C. similarly laying lengths of the wire along the array, within         every inter-tile groove, preferably using insulation sleeving         where perpendicular wires cross;     -   D. and in both cases leaving sufficient protruding wire at both         ends for electrical connection to be made and for subsequent         ties to be made;     -   E. preferably applying a row of relatively strong clips (such as         those of FIG. 3) across the joint or seam to be made so that a         force tending to compress the joint is applied to it during the         actual welding process and subsequent cooling,     -   F. applying an electric current through each wire in order to         cause resistive heating for the wire sufficient to reach a         welding temperature, and holding the temperature for an         effective duration, (see previously) so that tiles become welded         by their edges to adjacent tiles,     -   G. allowing the weld to cool before removing the clips.

The additional step of tying down the free ends of the wire to ground anchors (retainers) is not a necessary step, but if done is according to the following extra steps:

-   -   A. exteriorising the ends of each wire and about 250 mm should         be available for each wire     -   B. coupling each free end of the wire to a retainer (502)         attached to the prepared surface or the adjacent ground,     -   C. and applying controlled tension to or with the coupling which         may be a turnbuckle (501 in FIG. 5).

If the wire is not to be used with anchors or retainers it may be clipped off flush with the edges of the tiles or inserted into crimped-on covers in order to cover the sharp ends.

VARIATIONS

Although this description relates to working with resilient PVC playground tiles that include plasticiser, a similar approach may be applied to other structures made of similar plastics especially those that are unsuitable for gluing or RF heating. This may include flooring or roofing, for example.

The process may be used to vulcanise glued joints between rubber or predominantly rubber (such as rubber with urethane mixtures) tiles, and can also be used to weld synthetic rubber.

Some other plastics, blends and mixtures that may be used for tiles (apart from the PVC that forms the majority of examples considered herein) include (as a non-limiting list): high-density polyethylene (HDPE), ethylene copolymers with vinyl acetate (EVA), polyurethane (PUR), rubber, and other elastomers, or blends thereof. There are many further possible elastomers, for instance.

INDUSTRIAL APPLICABILITY AND ADVANTAGES

As mentioned previously in this specification, there have been problems with resilient floor tiles and play tiles coming apart or coming off their substrate especially as a result of temperature excursions (20 deg F. in winter to over 120 deg F. in the summer sun). The use of a welding process provides superior tile—tile edge adherence because glue is susceptible to later becoming softened by diffusing plasticiser. The inclusion of tension-bearing elements which can be secured to lateral supports such as pegs in the ground assists in keeping the array of tiles flat and in place despite expansion and contraction.

Finally, it will be understood that the scope of this invention as described and/or illustrated herein is not limited to the specified embodiments. Those of skill will appreciate that various modifications, additions, known equivalents, and substitutions are possible without departing from the scope and spirit of the invention as set forth in the following claims. 

1. A method for joining together flexible, resilient tiles having complementary edge configurations, comprising the steps of: positioning a plurality of the tiles in an array with complementary edges aligned; extending an elongate, electrically conductive, sacrificial member with tensile strength greater than the material of the tiles, along and between the complementary edges; pressing the tiles together so that complementary pairs of edges are abutted together with the conductive member engaged between them; and heating the conductive member to melt the material of the tiles along the abutted complementary edges and thereby weld the edges together.
 2. A method as claimed in claim 1, wherein the conductive member is heated to a temperature in the range from 140° C. to 180° C.
 3. A method as claimed in claim 1, further including the step of connecting the conductive member to an electricity supply and control means, whereby the conductive member is heated by resistance to an electric current passed through it.
 4. A method as claimed in claim 3, wherein the electric current produces a power input in the range from about 50 to 150 watts per metre length of the conductive member.
 5. A method as claimed in claim 3, wherein the conductive member is flexible, and is comprised of at least one wire.
 6. A method as claimed in claim 5, wherein the wire is formed from a stainless steel.
 7. A method as claimed in claim 5, wherein the wire is comprised of two or more entwined strands.
 8. A method as claimed in claim 1, wherein the conductive member is arranged with a section projecting beyond the edges of the tiles at both ends; further including the step of attaching the projecting sections to anchoring means; whereby the array of tiles can be anchored to a substrate by the anchoring means and the conductive member.
 9. A method as claimed in claim 1, wherein the step of pressing the tiles together is achieved by means including a clamp comprised of at least one pair of tile-engaging elements movably mounted with respect to each other on a support structure, and a drive mechanism by which the paired tile-engaging elements can be moved with respect to each other; further including the step of applying the clamp across adjoining tiles with the tile-engaging elements of at least one pair being each engaged with a different said tile; and the step of operating the drive mechanism to move the tile-engaging elements closer together, to press the tiles together.
 10. A method as claimed in claim 9, wherein the tiles have apertures therein, and the tile-engaging elements comprise projections adapted to fit into the apertures; and the step of applying the clamp across adjoining tiles includes engaging the projections in the apertures.
 11. A method as claimed in claim 1, wherein supplementary heat for remedying local deficiencies is obtained by use of a heated metal block.
 12. Apparatus for performing the method of claim 1, including: a plurality of flexible and resilient tiles having upper and lower surfaces and a plurality of edges such that apposed edges of said tiles can be aligned in order to provide substantially continuous conjoined upper and lower surfaces; an elongate, flexible electrically conductive member of a length greater than an edge of the tiles, with tensile strength greater than the material of the tiles; and means to heat the conductive member.
 13. Apparatus as claimed in claim 12, wherein the plurality of flexible and resilient tiles have complementary, configurations such that edges of two said tiles can be aligned and adjoined to provide substantially continuous conjoined upper and lower surfaces
 14. Apparatus as claimed in claim 12, wherein the conductive member is comprised of stainless steel wire.
 15. Apparatus as claimed in claim 12, wherein the conductive member is a wire comprised of two or more entwined strands.
 16. Apparatus as claimed in claim 12, wherein the means to heat the conductive member comprises an electricity supply and control means, and connecting means adapted to connect to the conductive member so as to pass a voltage-controlled and time-controlled electric current through the member, and includes means for indicating the delivered voltage and current.
 17. Apparatus as claimed in claim 16, wherein the electricity supply includes means for controlling the time of delivery and displaying a recommended cooling time.
 18. Apparatus as claimed in claim 12, further including a clamp comprised of at least one pair of tile-engaging elements movably mounted with respect to each other on a support structure, and a drive mechanism by which the paired tile-engaging elements can be moved with respect to each other.
 19. Apparatus as claimed in claim 18, wherein the drive mechanism includes a lever adapted and arranged to move at least one said tile-engaging element.
 20. Apparatus as claimed in claim 18, wherein the tiles have apertures in at least one said surface, and the tile-engaging elements comprise projections shaped and arranged to fit into and engage with the apertures.
 21. Apparatus as claimed in claim 18, further including a plurality of anchors each adapted to be fixed into a substrate and to be attached to an adjacent end of the conductive member. 